More graceful handling of no documentable libraries (#1839)

* More graceful handling of no documentable libraries

* flutter_plugin_tools has no libraries and will always fail

* Error message tweak and travis update

* Update test package docs for 2.1 release

Author: jcollins-g

lib/dartdoc.dart

@@ -178,7 +178,7 @@ class Dartdoc extends PackageBuilder {
     DartdocResults dartdocResults = await generateDocsBase();
     if (dartdocResults.packageGraph.publicLibraries.isEmpty) {
       throw new DartdocFailure(
-          "dartdoc could not find any libraries to document. Run `pub get` and try again.");
+          "dartdoc could not find any libraries to document");
     }
 
     final int errorCount =

lib/src/model.dart

@@ -6037,7 +6037,7 @@ class Package extends LibraryContainer
 
   /// Is this the package at the top of the list?  We display the first
   /// package specially (with "Libraries" rather than the package name).
-  bool get isFirstPackage => identical(packageGraph.localPackages.first, this);
+  bool get isFirstPackage => packageGraph.localPackages.isNotEmpty && identical(packageGraph.localPackages.first, this);
 
   @override
   bool get isSdk => packageMeta.isSdk;